The present invention relates to a low wear articulated buoy universal joint and buoy incorporating the joint. More particularly, the present invention relates to a universal buoy joint which is relatively inexpensive, can be easily incorporated into existing buoy designs, will have essentially zero-wear and will reduce the rotation about the buoy axis such that the markers will deviate less from the desired orientation. Furthermore, the present invention relates to a joint which is reliable during long term operation.
Articulated buoys have been developed to more accurately mark ship navigation channels and thereby reduce the channel width which must be dredged. Such buoys comprise a concrete sinker or anchor, a universal joint and a column. The column includes a buoyancy float at the top and a counterweight at the bottom. Without the counterweight, the large static uplift force produced by the buoyancy float would be transferred to the universal joint. The articulated buoy universal joint must accommodate a concrete sinker or anchor of significant weight hanging on the joint during installation. The joint must also accommodate joint loads and motions which occur during long periods of service. Furthermore, the joint must be fabricated of materials which will survive for long periods in a typical harbor or channel environment.
Universal joints on articulated buoys are known in which the joint is of the shackle type. Such a buoy is shown in the U.S. Pat. No. 3,800,347. According to this patent, the universal joint is created by linking two U-shape members together. One of the U-shape members is connected to the sinker and the other to the column of the buoy. While this type of joint is inexpensive to fabricate, it wears quickly. Wear on the joint is due to high contract (Hertz) stresses, constant working of the joint produced by buoy pitch motion, and abrasion by particles in the water near the channel floor. Wear also results from reversals in vertical load direction and rotations about the column axis. Field experience has shown that this type of joint lasts between 1 and 3 years. Such a short life span is undesirable in articulated buoys because replacement of the joint involves removal of the entire buoy. This is both time consuming and expensive.
Other types of joint designs have also been proposed for use in articulated buoys. Such designs include a simple section of flexible hose, universal ball and seat type bearings and mechanical joints. A mechanical ball and seat-type joint is also shown in U.S. Pat. No. 3,800,347. None of the above alternative joints is entirely satisfactory.
Mechanical joints involve close tolerance parts, making them relatively expensive. Mechanical joints require expensive materials, complex fabrication processes, and redundancy of systems to keep salt water and abrasive contaminants from between wearing surfaces. Mechanical joints also require extensive testing programs to confirm long term performance.
One type of mechanical joint commonly proposed for use in articulated buoys is a so-called Hooke's type joint with mechanical bearings. U.S. Pat. No. 4,037,273 shows such a Hooke's type joint. Such a joint tends to be expensive and still sensitive to wear unless carefully sealed. Another mechanical joint of the type proposed for use in articulated buoys is described in U.S. Pat. No. 3,666,395. This design is also expensive and sensitive to wear.
Hose-type joints comprise a short segment of hose installed between the column and the sinker in such a manner that the column pitch would be accommodated by bending of the hose. These joints are not subject to the same type of wear as mechanical joints, but are subject to buckling in the event that downward loads are produced due to wave action or absorption of water by the buoyancy float. Operating experience on offshore loading terminals has shown that hose life can be as short as 1-2 years in high cyclic loading environments.
Ball and seat type universal joints also involve close tolerance of parts, expensive materials, complex fabrication processes, and require expensive seal systems to keep salt water and abrasive contaminants from between the wearing surfaces. A ball and seat type joint for articulated buoys is described in U.S. Pat. No. 3,800,347.
Pat. No. 4,451,170, FIGS. 6 and 7, shows a ball and seat type universal joints with an elastomeric material between the ball and seat to accommodate joint rotation without wear. However, the joint shown is expensive to fabricate and will not permit 90.degree. rotations which could occur during buoy installation or if a buoy were to be run over by a barge etc. during operation.
It is therefore an object of the present invention to provide a low-wear articulated buoy joint which is relatively inexpensive. It is also an object of the present invention to provide a joint which can be easily incorporated into existing buoy designs. It is a further object of the present invention to provide a joint which will have essentially zero-wear. A still further object of the present invention is to provide a joint which will reduce the rotation about the buoy axis so that the markers will deviate less from the desired orientation.